Painting
Paint is a liquid which dries to form a surface. It is used in Engineering for protection, decoration and identification of
surfaces.
Paint consists mainly of:
- Pigments, which provide opacity and colour and can affect the degree of gloss
- Binders, which keep the powdery pigment together .
- Solvents (and thinners and dispersing agents), which evaporate after application to leave behind the film of bound pigment.
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When painting surfaces a "paint system" is generally used including the following stages:
Surface preparation
Application of a Primer layer.
Application of an Undercoat layer.
Application of a topcoat.
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It is preferable to obtain the primer, Undercoat and the topcoat from the same manufacturer.
A Paint can be either water based or solvent based.
In a solvent based paint a solvent is used with the binder to obtain suitable consistency for applying the paint.
The solvent evaporates after application. The binder forms a film which hardens generally as a result of chemical reaction .
In production processes the hardening/ curing process is accelerated using additional processes including stoving in special ovens.
In the curing process the binder changes chemically and is no long soluble in the solvent.
Laquers are solvent based paints that simply harden as a result of the solvent evaporation with no chemical change
to the film.
Water based paints are either
Emulusions of polymers depending on for their drying properties on the evaporation of water and coalescing of the polymer particles.
Water soluble resins which are applied by electro-deposition or by dipping followed by stoving.
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Galvanising
Galvanising is a method of protecting a steel surface from corrosion by providing
a surface coating of Zinc. Galvanised coatings corrode preferentially to steel,
providing sacrificial or cathodic protection to small areas of steel exposed through
damage to the coating. Unlike organic coatings no touch up is needed.
The galvanising process involves the following stages:
- Any surface oil or grease is removed by suitable degreasing agents.
- The steel is then usually cleaned of all rust and scale by
acid pickling. This may be preceded by blast cleaning to
remove scale and roughen the surface but such surfaces are
always subsequently pickled in inhibited hydrochloric acid.
- The cleaned steel is then immersed in a fluxing agent to
ensure good contact between the steel and zinc during the galvanising process.
- The cleaned and fluxed steel is dipped into a bath of molten zinc at a
temperature of about 450oC. At this temperature, the steel reacts with
the molten zinc to form a series of zinc/iron alloys integral with the steel surface.
- As the steel workpiece is removed from the bath, a layer of relatively pure
zinc is deposited on top of the alloy layers.
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